1. Field of the Invention
The present invention relates to the analysis of underground earth formations, and, more particularly, to the determination of formation resistivity.
2. Description of the Related Art
Boreholes are drilled into the earth for many applications such as hydrocarbon production, geothermal production, and carbon sequestration. In order to efficiently use expensive resources drilling the boreholes, it is important for analysts to acquire detailed information related to the geologic formations being drilled.
Resistivity imaging is one type of process for obtaining the detailed information. In resistivity imaging, both electrical and induction resistivity instruments can be used. The resistivity of a formation is measured as a function of depth using a resistivity tool disposed in a borehole penetrating the formation. Variations in the resistivity are plotted or displayed to provide an image of the formation.
In electrical resistivity imaging, one or more transmitter electrodes are used to inject an electric current into an earth formation. Measurement electrodes, sometimes referred to as button electrodes, sink these currents and perform electrical measurements that are used to determine the resistivity of the earth formation. Because the transmitter and transmitter electrodes are deployed in a drilled borehole having variations in diameter due to the drilling process, the electrodes may not make contact with the borehole wall. The space or distance between an electrode and the borehole wall is referred to as the “standoff.” Variations in the standoff could negatively affect quality of acquired resistivity images. Moreover, when using oil-based drilling mud, the drilling mud may enter a standoff and this could make measurement conditions even worse and result in erratic images. It would be well received in the art if the quality of resistivity images could be improved when using oil-based drilling mud.